Object


1   /*
2    * @(#)Object.java  1.68 04/04/08
3    *
4    * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
5    * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6    */
7   
8   package java.lang;
9   
10  /**
11   * Class <code>Object</code> is the root of the class hierarchy. 
12   * Every class has <code>Object</code> as a superclass. All objects, 
13   * including arrays, implement the methods of this class. 
14   *
15   * @author  unascribed
16   * @version 1.68, 04/08/04
17   * @see     java.lang.Class
18   * @since   JDK1.0
19   */
20  public class Object {
21  
22      private static native void registerNatives();
23      static {
24          registerNatives();
25      }
26  
27      /**
28       * Returns the runtime class of an object. That <tt>Class</tt> 
29       * object is the object that is locked by <tt>static synchronized</tt> 
30       * methods of the represented class.
31       *
32       * @return The <code>java.lang.Class</code> object that represents
33       *         the runtime class of the object.  The result is of type
34       *         {@code Class<? extends X>} where X is the
35       *         erasure of the static type of the expression on which
36       *         <code>getClass</code> is called.
37       */
38      public final native Class  <? extends Object  > getClass();
39  
40      /**
41       * Returns a hash code value for the object. This method is 
42       * supported for the benefit of hashtables such as those provided by 
43       * <code>java.util.Hashtable</code>. 
44       * <p>
45       * The general contract of <code>hashCode</code> is: 
46       * <ul>
47       * <li>Whenever it is invoked on the same object more than once during 
48       *     an execution of a Java application, the <tt>hashCode</tt> method 
49       *     must consistently return the same integer, provided no information 
50       *     used in <tt>equals</tt> comparisons on the object is modified.
51       *     This integer need not remain consistent from one execution of an
52       *     application to another execution of the same application. 
53       * <li>If two objects are equal according to the <tt>equals(Object)</tt>
54       *     method, then calling the <code>hashCode</code> method on each of 
55       *     the two objects must produce the same integer result. 
56       * <li>It is <em>not</em> required that if two objects are unequal 
57       *     according to the {@link java.lang.Object#equals(java.lang.Object)} 
58       *     method, then calling the <tt>hashCode</tt> method on each of the 
59       *     two objects must produce distinct integer results.  However, the 
60       *     programmer should be aware that producing distinct integer results 
61       *     for unequal objects may improve the performance of hashtables.
62       * </ul>
63       * <p>
64       * As much as is reasonably practical, the hashCode method defined by 
65       * class <tt>Object</tt> does return distinct integers for distinct 
66       * objects. (This is typically implemented by converting the internal 
67       * address of the object into an integer, but this implementation 
68       * technique is not required by the 
69       * Java<font size="-2"><sup>TM</sup></font> programming language.)
70       *
71       * @return  a hash code value for this object.
72       * @see     java.lang.Object#equals(java.lang.Object)
73       * @see     java.util.Hashtable
74       */
75      public native int hashCode();
76  
77      /**
78       * Indicates whether some other object is "equal to" this one.
79       * <p>
80       * The <code>equals</code> method implements an equivalence relation
81       * on non-null object references:
82       * <ul>
83       * <li>It is <i>reflexive</i>: for any non-null reference value
84       *     <code>x</code>, <code>x.equals(x)</code> should return
85       *     <code>true</code>.
86       * <li>It is <i>symmetric</i>: for any non-null reference values
87       *     <code>x</code> and <code>y</code>, <code>x.equals(y)</code>
88       *     should return <code>true</code> if and only if
89       *     <code>y.equals(x)</code> returns <code>true</code>.
90       * <li>It is <i>transitive</i>: for any non-null reference values
91       *     <code>x</code>, <code>y</code>, and <code>z</code>, if
92       *     <code>x.equals(y)</code> returns <code>true</code> and
93       *     <code>y.equals(z)</code> returns <code>true</code>, then
94       *     <code>x.equals(z)</code> should return <code>true</code>.
95       * <li>It is <i>consistent</i>: for any non-null reference values
96       *     <code>x</code> and <code>y</code>, multiple invocations of
97       *     <tt>x.equals(y)</tt> consistently return <code>true</code>
98       *     or consistently return <code>false</code>, provided no
99       *     information used in <code>equals</code> comparisons on the
100      *     objects is modified.
101      * <li>For any non-null reference value <code>x</code>,
102      *     <code>x.equals(null)</code> should return <code>false</code>.
103      * </ul>
104      * <p>
105      * The <tt>equals</tt> method for class <code>Object</code> implements 
106      * the most discriminating possible equivalence relation on objects; 
107      * that is, for any non-null reference values <code>x</code> and
108      * <code>y</code>, this method returns <code>true</code> if and only
109      * if <code>x</code> and <code>y</code> refer to the same object
110      * (<code>x == y</code> has the value <code>true</code>).
111      * <p>
112      * Note that it is generally necessary to override the <tt>hashCode</tt>
113      * method whenever this method is overridden, so as to maintain the
114      * general contract for the <tt>hashCode</tt> method, which states
115      * that equal objects must have equal hash codes. 
116      *
117      * @param   obj   the reference object with which to compare.
118      * @return  <code>true</code> if this object is the same as the obj
119      *          argument; <code>false</code> otherwise.
120      * @see     #hashCode()
121      * @see     java.util.Hashtable
122      */
123     public boolean equals(Object   obj) {
124     return (this == obj);
125     }
126 
127     /**
128      * Creates and returns a copy of this object.  The precise meaning 
129      * of "copy" may depend on the class of the object. The general 
130      * intent is that, for any object <tt>x</tt>, the expression:
131      * <blockquote>
132      * <pre>
133      * x.clone() != x</pre></blockquote>
134      * will be true, and that the expression:
135      * <blockquote>
136      * <pre>
137      * x.clone().getClass() == x.getClass()</pre></blockquote>
138      * will be <tt>true</tt>, but these are not absolute requirements. 
139      * While it is typically the case that:
140      * <blockquote>
141      * <pre>
142      * x.clone().equals(x)</pre></blockquote>
143      * will be <tt>true</tt>, this is not an absolute requirement. 
144      * <p>
145      * By convention, the returned object should be obtained by calling
146      * <tt>super.clone</tt>.  If a class and all of its superclasses (except
147      * <tt>Object</tt>) obey this convention, it will be the case that
148      * <tt>x.clone().getClass() == x.getClass()</tt>.
149      * <p>
150      * By convention, the object returned by this method should be independent
151      * of this object (which is being cloned).  To achieve this independence,
152      * it may be necessary to modify one or more fields of the object returned
153      * by <tt>super.clone</tt> before returning it.  Typically, this means
154      * copying any mutable objects that comprise the internal "deep structure"
155      * of the object being cloned and replacing the references to these
156      * objects with references to the copies.  If a class contains only
157      * primitive fields or references to immutable objects, then it is usually
158      * the case that no fields in the object returned by <tt>super.clone</tt>
159      * need to be modified.
160      * <p>
161      * The method <tt>clone</tt> for class <tt>Object</tt> performs a 
162      * specific cloning operation. First, if the class of this object does 
163      * not implement the interface <tt>Cloneable</tt>, then a 
164      * <tt>CloneNotSupportedException</tt> is thrown. Note that all arrays 
165      * are considered to implement the interface <tt>Cloneable</tt>. 
166      * Otherwise, this method creates a new instance of the class of this 
167      * object and initializes all its fields with exactly the contents of 
168      * the corresponding fields of this object, as if by assignment; the
169      * contents of the fields are not themselves cloned. Thus, this method 
170      * performs a "shallow copy" of this object, not a "deep copy" operation.
171      * <p>
172      * The class <tt>Object</tt> does not itself implement the interface 
173      * <tt>Cloneable</tt>, so calling the <tt>clone</tt> method on an object 
174      * whose class is <tt>Object</tt> will result in throwing an
175      * exception at run time.
176      *
177      * @return     a clone of this instance.
178      * @exception  CloneNotSupportedException  if the object's class does not
179      *               support the <code>Cloneable</code> interface. Subclasses
180      *               that override the <code>clone</code> method can also
181      *               throw this exception to indicate that an instance cannot
182      *               be cloned.
183      * @see java.lang.Cloneable
184      */
185     protected native Object   clone() throws CloneNotSupportedException  ;
186 
187     /**
188      * Returns a string representation of the object. In general, the 
189      * <code>toString</code> method returns a string that 
190      * "textually represents" this object. The result should 
191      * be a concise but informative representation that is easy for a 
192      * person to read.
193      * It is recommended that all subclasses override this method.
194      * <p>
195      * The <code>toString</code> method for class <code>Object</code> 
196      * returns a string consisting of the name of the class of which the 
197      * object is an instance, the at-sign character `<code>@</code>', and 
198      * the unsigned hexadecimal representation of the hash code of the 
199      * object. In other words, this method returns a string equal to the 
200      * value of:
201      * <blockquote>
202      * <pre>
203      * getClass().getName() + '@' + Integer.toHexString(hashCode())
204      * </pre></blockquote>
205      *
206      * @return  a string representation of the object.
207      */
208     public String   toString() {
209     return getClass().getName() + "@" + Integer.toHexString(hashCode());
210     }
211 
212     /**
213      * Wakes up a single thread that is waiting on this object's 
214      * monitor. If any threads are waiting on this object, one of them 
215      * is chosen to be awakened. The choice is arbitrary and occurs at 
216      * the discretion of the implementation. A thread waits on an object's 
217      * monitor by calling one of the <code>wait</code> methods.
218      * <p>
219      * The awakened thread will not be able to proceed until the current 
220      * thread relinquishes the lock on this object. The awakened thread will 
221      * compete in the usual manner with any other threads that might be 
222      * actively competing to synchronize on this object; for example, the 
223      * awakened thread enjoys no reliable privilege or disadvantage in being 
224      * the next thread to lock this object.
225      * <p>
226      * This method should only be called by a thread that is the owner 
227      * of this object's monitor. A thread becomes the owner of the 
228      * object's monitor in one of three ways: 
229      * <ul>
230      * <li>By executing a synchronized instance method of that object. 
231      * <li>By executing the body of a <code>synchronized</code> statement 
232      *     that synchronizes on the object. 
233      * <li>For objects of type <code>Class,</code> by executing a 
234      *     synchronized static method of that class. 
235      * </ul>
236      * <p>
237      * Only one thread at a time can own an object's monitor. 
238      *
239      * @exception  IllegalMonitorStateException  if the current thread is not
240      *               the owner of this object's monitor.
241      * @see        java.lang.Object#notifyAll()
242      * @see        java.lang.Object#wait()
243      */
244     public final native void notify();
245 
246     /**
247      * Wakes up all threads that are waiting on this object's monitor. A 
248      * thread waits on an object's monitor by calling one of the 
249      * <code>wait</code> methods.
250      * <p>
251      * The awakened threads will not be able to proceed until the current 
252      * thread relinquishes the lock on this object. The awakened threads 
253      * will compete in the usual manner with any other threads that might 
254      * be actively competing to synchronize on this object; for example, 
255      * the awakened threads enjoy no reliable privilege or disadvantage in 
256      * being the next thread to lock this object.
257      * <p>
258      * This method should only be called by a thread that is the owner 
259      * of this object's monitor. See the <code>notify</code> method for a 
260      * description of the ways in which a thread can become the owner of 
261      * a monitor. 
262      *
263      * @exception  IllegalMonitorStateException  if the current thread is not
264      *               the owner of this object's monitor.
265      * @see        java.lang.Object#notify()
266      * @see        java.lang.Object#wait()
267      */
268     public final native void notifyAll();
269 
270     /**
271      * Causes current thread to wait until either another thread invokes the 
272      * {@link java.lang.Object#notify()} method or the 
273      * {@link java.lang.Object#notifyAll()} method for this object, or a 
274      * specified amount of time has elapsed. 
275      * <p>
276      * The current thread must own this object's monitor. 
277      * <p>
278      * This method causes the current thread (call it <var>T</var>) to 
279      * place itself in the wait set for this object and then to relinquish 
280      * any and all synchronization claims on this object. Thread <var>T</var> 
281      * becomes disabled for thread scheduling purposes and lies dormant 
282      * until one of four things happens:
283      * <ul>
284      * <li>Some other thread invokes the <tt>notify</tt> method for this 
285      * object and thread <var>T</var> happens to be arbitrarily chosen as 
286      * the thread to be awakened. 
287      * <li>Some other thread invokes the <tt>notifyAll</tt> method for this 
288      * object. 
289      * <li>Some other thread {@link java.lang.Thread#interrupt() interrupts} 
290      * thread <var>T</var>. 
291      * <li>The specified amount of real time has elapsed, more or less.  If 
292      * <tt>timeout</tt> is zero, however, then real time is not taken into 
293      * consideration and the thread simply waits until notified. 
294      * </ul>
295      * The thread <var>T</var> is then removed from the wait set for this 
296      * object and re-enabled for thread scheduling. It then competes in the 
297      * usual manner with other threads for the right to synchronize on the 
298      * object; once it has gained control of the object, all its 
299      * synchronization claims on the object are restored to the status quo 
300      * ante - that is, to the situation as of the time that the <tt>wait</tt> 
301      * method was invoked. Thread <var>T</var> then returns from the 
302      * invocation of the <tt>wait</tt> method. Thus, on return from the 
303      * <tt>wait</tt> method, the synchronization state of the object and of 
304      * thread <tt>T</tt> is exactly as it was when the <tt>wait</tt> method 
305      * was invoked. 
306      * <p>
307      * A thread can also wake up without being notified, interrupted, or
308      * timing out, a so-called <i>spurious wakeup</i>.  While this will rarely
309      * occur in practice, applications must guard against it by testing for
310      * the condition that should have caused the thread to be awakened, and
311      * continuing to wait if the condition is not satisfied.  In other words,
312      * waits should always occur in loops, like this one:
313      * <pre>
314      *     synchronized (obj) {
315      *         while (&lt;condition does not hold&gt;)
316      *             obj.wait(timeout);
317      *         ... // Perform action appropriate to condition
318      *     }
319      * </pre>
320      * (For more information on this topic, see Section 3.2.3 in Doug Lea's
321      * "Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
322      * 2000), or Item 50 in Joshua Bloch's "Effective Java Programming
323      * Language Guide" (Addison-Wesley, 2001).
324      * <p>
325      * If the current thread is 
326      * {@link java.lang.Thread#interrupt() interrupted} by another thread 
327      * while it is waiting, then an <tt>InterruptedException</tt> is thrown. 
328      * This exception is not thrown until the lock status of this object has 
329      * been restored as described above.
330      * <p>
331      * Note that the <tt>wait</tt> method, as it places the current thread 
332      * into the wait set for this object, unlocks only this object; any 
333      * other objects on which the current thread may be synchronized remain 
334      * locked while the thread waits.
335      * <p>
336      * This method should only be called by a thread that is the owner 
337      * of this object's monitor. See the <code>notify</code> method for a 
338      * description of the ways in which a thread can become the owner of 
339      * a monitor. 
340      *
341      * @param      timeout   the maximum time to wait in milliseconds.
342      * @exception  IllegalArgumentException      if the value of timeout is
343      *           negative.
344      * @exception  IllegalMonitorStateException  if the current thread is not
345      *               the owner of the object's monitor.
346      * @exception  InterruptedException if another thread interrupted the
347      *             current thread before or while the current thread
348      *             was waiting for a notification.  The <i>interrupted
349      *             status</i> of the current thread is cleared when
350      *             this exception is thrown.
351      * @see        java.lang.Object#notify()
352      * @see        java.lang.Object#notifyAll()
353      */
354     public final native void wait(long timeout) throws InterruptedException  ;
355 
356     /**
357      * Causes current thread to wait until another thread invokes the 
358      * {@link java.lang.Object#notify()} method or the 
359      * {@link java.lang.Object#notifyAll()} method for this object, or 
360      * some other thread interrupts the current thread, or a certain 
361      * amount of real time has elapsed. 
362      * <p>
363      * This method is similar to the <code>wait</code> method of one 
364      * argument, but it allows finer control over the amount of time to 
365      * wait for a notification before giving up. The amount of real time, 
366      * measured in nanoseconds, is given by:
367      * <blockquote>
368      * <pre>
369      * 1000000*timeout+nanos</pre></blockquote>
370      * <p>
371      * In all other respects, this method does the same thing as the 
372      * method {@link #wait(long)} of one argument. In particular, 
373      * <tt>wait(0, 0)</tt> means the same thing as <tt>wait(0)</tt>.
374      * <p>
375      * The current thread must own this object's monitor. The thread 
376      * releases ownership of this monitor and waits until either of the 
377      * following two conditions has occurred: 
378      * <ul>
379      * <li>Another thread notifies threads waiting on this object's monitor 
380      *     to wake up either through a call to the <code>notify</code> method 
381      *     or the <code>notifyAll</code> method. 
382      * <li>The timeout period, specified by <code>timeout</code> 
383      *     milliseconds plus <code>nanos</code> nanoseconds arguments, has 
384      *     elapsed. 
385      * </ul>
386      * <p>
387      * The thread then waits until it can re-obtain ownership of the 
388      * monitor and resumes execution.
389      * <p>
390      * As in the one argument version, interrupts and spurious wakeups are
391      * possible, and this method should always be used in a loop:
392      * <pre>
393      *     synchronized (obj) {
394      *         while (&lt;condition does not hold&gt;)
395      *             obj.wait(timeout, nanos);
396      *         ... // Perform action appropriate to condition
397      *     }
398      * </pre>
399      * This method should only be called by a thread that is the owner 
400      * of this object's monitor. See the <code>notify</code> method for a 
401      * description of the ways in which a thread can become the owner of 
402      * a monitor. 
403      *
404      * @param      timeout   the maximum time to wait in milliseconds.
405      * @param      nanos      additional time, in nanoseconds range
406      *                       0-999999.
407      * @exception  IllegalArgumentException      if the value of timeout is
408      *              negative or the value of nanos is
409      *              not in the range 0-999999.
410      * @exception  IllegalMonitorStateException  if the current thread is not
411      *               the owner of this object's monitor.
412      * @exception  InterruptedException if another thread interrupted the
413      *             current thread before or while the current thread
414      *             was waiting for a notification.  The <i>interrupted
415      *             status</i> of the current thread is cleared when
416      *             this exception is thrown.
417      */
418     public final void wait(long timeout, int nanos) throws InterruptedException   {
419         if (timeout < 0) {
420             throw new IllegalArgumentException  ("timeout value is negative");
421         }
422 
423         if (nanos < 0 || nanos > 999999) {
424             throw new IllegalArgumentException  (
425                 "nanosecond timeout value out of range");
426         }
427 
428     if (nanos >= 500000 || (nanos != 0 && timeout == 0)) {
429         timeout++;
430     }
431 
432     wait(timeout);
433     }
434 
435     /**
436      * Causes current thread to wait until another thread invokes the 
437      * {@link java.lang.Object#notify()} method or the 
438      * {@link java.lang.Object#notifyAll()} method for this object. 
439      * In other words, this method behaves exactly as if it simply 
440      * performs the call <tt>wait(0)</tt>.
441      * <p>
442      * The current thread must own this object's monitor. The thread 
443      * releases ownership of this monitor and waits until another thread 
444      * notifies threads waiting on this object's monitor to wake up 
445      * either through a call to the <code>notify</code> method or the 
446      * <code>notifyAll</code> method. The thread then waits until it can 
447      * re-obtain ownership of the monitor and resumes execution. 
448      * <p>
449      * As in the one argument version, interrupts and spurious wakeups are
450      * possible, and this method should always be used in a loop:
451      * <pre>
452      *     synchronized (obj) {
453      *         while (&lt;condition does not hold&gt;)
454      *             obj.wait();
455      *         ... // Perform action appropriate to condition
456      *     }
457      * </pre>
458      * This method should only be called by a thread that is the owner 
459      * of this object's monitor. See the <code>notify</code> method for a 
460      * description of the ways in which a thread can become the owner of 
461      * a monitor. 
462      *
463      * @exception  IllegalMonitorStateException  if the current thread is not
464      *               the owner of the object's monitor.
465      * @exception  InterruptedException if another thread interrupted the
466      *             current thread before or while the current thread
467      *             was waiting for a notification.  The <i>interrupted
468      *             status</i> of the current thread is cleared when
469      *             this exception is thrown.
470      * @see        java.lang.Object#notify()
471      * @see        java.lang.Object#notifyAll()
472      */
473     public final void wait() throws InterruptedException   {
474     wait(0);
475     }
476 
477     /**
478      * Called by the garbage collector on an object when garbage collection
479      * determines that there are no more references to the object.
480      * A subclass overrides the <code>finalize</code> method to dispose of
481      * system resources or to perform other cleanup. 
482      * <p>
483      * The general contract of <tt>finalize</tt> is that it is invoked 
484      * if and when the Java<font size="-2"><sup>TM</sup></font> virtual 
485      * machine has determined that there is no longer any
486      * means by which this object can be accessed by any thread that has
487      * not yet died, except as a result of an action taken by the
488      * finalization of some other object or class which is ready to be
489      * finalized. The <tt>finalize</tt> method may take any action, including
490      * making this object available again to other threads; the usual purpose
491      * of <tt>finalize</tt>, however, is to perform cleanup actions before 
492      * the object is irrevocably discarded. For example, the finalize method 
493      * for an object that represents an input/output connection might perform
494      * explicit I/O transactions to break the connection before the object is
495      * permanently discarded. 
496      * <p>
497      * The <tt>finalize</tt> method of class <tt>Object</tt> performs no 
498      * special action; it simply returns normally. Subclasses of 
499      * <tt>Object</tt> may override this definition.
500      * <p>
501      * The Java programming language does not guarantee which thread will 
502      * invoke the <tt>finalize</tt> method for any given object. It is 
503      * guaranteed, however, that the thread that invokes finalize will not 
504      * be holding any user-visible synchronization locks when finalize is 
505      * invoked. If an uncaught exception is thrown by the finalize method, 
506      * the exception is ignored and finalization of that object terminates.
507      * <p>
508      * After the <tt>finalize</tt> method has been invoked for an object, no 
509      * further action is taken until the Java virtual machine has again 
510      * determined that there is no longer any means by which this object can 
511      * be accessed by any thread that has not yet died, including possible
512      * actions by other objects or classes which are ready to be finalized, 
513      * at which point the object may be discarded.
514      * <p>
515      * The <tt>finalize</tt> method is never invoked more than once by a Java
516      * virtual machine for any given object.
517      * <p>
518      * Any exception thrown by the <code>finalize</code> method causes 
519      * the finalization of this object to be halted, but is otherwise 
520      * ignored. 
521      *
522      * @throws Throwable the <code>Exception</code> raised by this method
523      */
524     protected void finalize() throws Throwable   { }
525 }
526
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